We integrated stomach content analysis (SCA) and stable isotope analysis (SIA) to understand ontogenetic niche shifts in the invasive freshwater fish, bluegill, Lepomis macrochirus, inhabiting the Yedang Reservoir in Korea. Based on the total length (TL), we classified L. macrochirus as small (23–57 mm), medium (61–99 mm), or large (100–163 mm). Across all study sites, the index of relative importance (IRI) of zooplankton was high for small individuals, whereas those of benthic macroinvertebrates were high for the medium and large groups. Isotopic niche width estimates based on carbon and nitrogen isotope ratios (δ space) also increased with growth, indicating an ontogenetic niche shift in L. macrochirus. In particular, the δ space and diet plasticity of large fish was higher in the littoral population, implying L. macrochirus are generalist feeders as adults. Individuals classified as small showed confined and constant δ space, regardless of habitat condition. Thus, together with the results on the significantly high IRI of zooplankton, these individuals seem to have strong specialistic feeding ecology. Our study demonstrates the applicability strength of combining SCA and SIA for ecological niche research by providing clear evidence of an ontogenetic niche shift in L. macrochirus and elucidates their feeding ecology.

Leiocassis longirostris, a large benthic catfish, has been classified as Regionally Extinct (RE) in South Korea since the 1970s. Assessing the success of ongoing restocking efforts in the Geum River is challenging due to limitations in traditional capture-based monitoring methods. We developed a highly sensitive, species-specific quantitative real-time PCR (qPCR) assay targeting the mitochondrial cytochrome b gene (mt-cyb) for this restocked fish. Assay validation confirmed high specificity against a panel of 25 co-occurring fish species, including 8 related siluriforms, and an excellent limit of detection of 10 copies per reaction. We applied this assay to environmental DNA (eDNA) extracted from paired water and sediment samples collected from 13 stations across the Geum River Basin. eDNA was detected at three water and five sediment stations. Notably, detection rates and average concentrations were consistently higher in a sediment sample (peaking at 1609.93 copies L⁻¹), an observation consistent with the species’ benthic ecology. These findings provide the first molecular evidence that restocked L. longirostris may disperse and persist in the mid to lower reaches of the Geum River. This validated eDNA qPCR tool is essential for guiding the integrated monitoring and adaptive management strategies required to support the long-term recovery of this ecologically important species. This study is the first species-specific qPCR of the bagrid catfish in South Korea.